As electronic equipment has become faster with clock speeds of hundreds
of MHz and above, even above 1 GHz, impulsive EMI events in the environment,
such as ESD, have become a major source of equipment malfunction. Events
involving collisions of small pieces of metal, such as coins, are an example
of one mechanism that can generate impulsive EMI.

To characterize the EMI generated by such events an antenna is needed
that can accurately reproduce the time domain waveform of a fast impulse.
Figure 1 shows a view looking into such an antenna, known as a TEM antenna.
Figure 2 shows a view of the complete antenna. This antenna is basically
a 50 Ohm tapered transmission line. A balun is located at the connection
point to the coaxial cable to interface the balanced signal from the antenna
to the unbalanced coaxial cable. As impulsive EMI becomes more important
with faster equipment, antenna structures like this will become more important
and will have a direct effect on circuit designs. Antennae like this one
are being used in standards development from which requirements will be
placed on circuit designs and their testing.

Figure 2. Overall View of TEM Antenna

The electric field strength is related to the antenna output by multiplying
the antenna output by 1 meter divided by the plate separation at the front
of the antenna. For the antenna in Figure 2, the antenna output is multiplied
by approximately 7 to get the electric field strength. Figure 3 shows the
output of the antenna resulting from the collision of two coins in front
of the antenna that are differentially charged by a few hundred volts.
Notice that one pulse appears, with no ringing. The electric field amplitude
is about 7/m x 2.5 Volts or about 17.5 Volts/meter, a significant signal.
Another way to think of the signal strength is that it delivered 1/8 watt
of peak power into the 50 Ohm load at the scope (2.5*2.5/50). Not bad for
a couple of coins!

Figure 3. TEM Antenna Output for Small Metal ESD

Figure 4 below shows the output of a dipole antenna approximately 30
cm in length near an ESD event. This picture appeared in my 1999 EOS/ESD
Symposium paper, Unusual
Forms of ESD and Their Effects. Most antenna structures
have one or more resonant frequencies and this gives them a ringing response
to an impulse. In addition, many antennas designed for use in the frequency
domain such as for EMC testing, have dispersive characteristics that distort
fast pulses. When one sees a response like Figure 4, the antenna is suspect
and must be checked. While the waveform of Figure 4 indicates the presence
of an ESD event and something about the available energy, it does not give
an accurate picture of the actual impulsive field that was radiated.

Figure 4. Dipole Antenna Output for Impulsive ESD

I am working with engineers from several companies to collect data using
a TEM antenna in typical environments, such as server installations. This
data will be published in the next year and it should prove useful to both
designers of equipment and the IT, information technology, people who maintain
it.